Apparatus for controlling the flow rate of an air seeder

ABSTRACT

An apparatus for dispensing particles includes a storage tank for containing particles to be dispensed a metering mechanism for transferring particles from the storage tank and an air distribution system for receiving particles from the metering mechanism. The air distribution system includes a bank of tubes for transporting particles in a flow of air having a flow rate, and a damper mechanism at the bank of tubes for selective control of the flow rate within the bank of tubes. The apparatus for dispensing particles also includes an air flow source for providing a flow of air to the bank of tubes, a sensor located at each tube for providing a sensor output signal indicative of a value of a flow characteristic, and a controller operably connected to the sensors and to the damper mechanism. The controller automatically effects an adjustment of the flow rate in the bank of tubes by receiving the sensor output signals, normalizing the values corresponding to the sensor output signals, and automatically effecting an adjustment of the damper mechanism as a function of the normalized values and a target value.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U. S. patent applicationSer. No. 09/316,136, filed May 21, 1999, now issued as U. S. Pat. No.5,996,516, on Dec. 7, 1999.

BACKGROUND OF THE INVENTION

The present invention relates generally to a system for dispensingparticles. In particular, the invention relates to an apparatus andmethod for monitoring and controlling the velocity of particles in apneumatic product application system.

Agricultural implements such as air seeders and row crop planters employpneumatic means to convey product such as seed and fertilizer from acentralized hopper to distribution on the ground. Controlling particleflow at an acceptable operating speed can be a difficult task. Too muchair velocity and thus increased particle velocity can result in productdamage and product bouncing or blowing out of the ground furrow. Highparticle velocity can also result in increased wear on the plastic airlines of an air seeder and excess consumption of fan power. Too littleair velocity can result in plugging of air delivery lines.

The air velocity is difficult to optimize for a number of reasons. Theminimum carry velocity varies for different product mass flow rates andfor different product types, both of which may change during a fieldoperation.

Another issue adding to the difficulty in optimizing the air flowvelocity occurs when the configuration of the cart is altered. Thedelivery hoses in the air system may be reconfigured in a number of waysupsetting the balance of the air flow system. Even during fieldoperation, if the air seeding system operates on a side slope, theresistance to particle flow will become greater in the line which haveraised and vice versa for the lower lines.

Prior art methods of monitoring the particle flow of a system generallycount the seeds at some point in the delivery system to ensure theproper number of seeds is being placed into the ground. Other systemssimply monitor the air flow velocity, which provides a generalindication of operating levels, but is not accurate since differentproducts have different carry velocities.

Many air carts are equipped with methods of adjusting the air flowthrough the delivery lines by means of damping mechanisms and fan speedadjustment. One such system is disclosed in United States provisionalpatent application Ser. No. 60/086,422. This prior art system requiresinteraction of the operator to increase or decrease the air velocity.

The drawbacks of prior art include the need for manual adjustments todamping mechanisms and fan speed, complexity of the manual adjustments,and lost time and money on operation and maintenance of the air carts.

There are also further difficulties when the air cart is configured fordouble or triple shoot applications since each product may have adifferent carrying velocity.

Thus, there is a need for a system of monitoring and controllingparticle velocity automatically in an air delivery system.

SUMMARY OF THE INVENTION

The present invention is directed to an apparatus and method fordispensing particles.

According to one aspect of the present invention, there is provided anapparatus for dispensing particles including: an air distribution systemincluding at least one tube for transporting air borne particles in aflow of air having a flow rate; an air flow source for providing a flowof air to the air distribution system; and a controller forautomatically effecting an adjustment of the flow rate.

According to a further aspect of the present invention, there isprovided an apparatus for dispensing particles including: a storage tankfor containing particles to be dispensed; a metering mechanism fortransferring particles from the storage tank; an air distribution systemfor receiving particles from the metering mechanism, the airdistribution system comprising: a bank of tubes for transportingparticles in a flow of air having a flow rate, and a damper mechanism atthe bank of tubes for selective control of the flow rate within the bankof tubes; an air flow source for providing a flow of air to the bank oftubes; a sensor located at each tube for providing a sensor outputsignal indicative of a value of a flow characteristic; and a controlleroperably connected to the sensors and to the damper mechanism such thatthe controller automatically effects an adjustment of the flow rate inthe bank of tubes by receiving the sensor output signals, normalizingthe values corresponding to the sensor output signals, and automaticallyeffecting an adjustment of the damper mechanism as a function of thenormalized values and a target value.

According to a further aspect of the present invention, there isprovided a method for controlling the velocity of particles in anapparatus for dispensing particles, the apparatus comprising: a storagetank for containing particles to be dispensed; a metering mechanism fortransferring particles from the storage tank; an air distribution systemfor receiving particles from the metering mechanism, the airdistribution system comprising a bank of tubes for transportingparticles in a flow of air having a flow rate, and a damper mechanism atthe bank of tubes for selective control of the flow rate within the bankof tubes; an air flow source for providing a flow of air to the bank oftubes; a particle velocity sensor located at each tube for providing asensor output signal corresponding to a measured particle velocity; anda controller operably connected to the particle velocity sensors and tothe damper mechanism for automatically controlling the velocity ofparticles in the bank of tubes. The method includes, at each particlevelocity sensor, the steps of: detecting particles at an upstreamlocation; detecting particles at a downstream location, the downstreamlocation being a known distance from the upstream location; determiningthe time interval between detection of the particles at the upstreamlocation and detection of the particles at the downstream location;dividing the known distance by the time interval to generate a particlevelocity value; providing a sensor output signal corresponding to theparticle velocity value. The method further includes, at the controller,the steps of: receiving the sensor output signals; normalizing theparticle velocity values corresponding to the sensor output signals; andautomatically effecting an adjustment of the damper mechanism as afunction of the normalized values and a target value.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages of this invention will be apparent upon consideration ofthe following detailed disclosure of the invention, especially whentaken in conjunction with the accompanying drawings wherein:

FIG. 1 is a side elevational view of an air seeder embodying the presentinvention;

FIG. 2 is a schematic cross sectional view through the tanks of the airseeder of FIG. 1;

FIG. 3 is a cross sectional view illustrating a single run of the airdistribution system of the embodiment of FIG. 1;

FIGS. 4a to 4 d illustrate the mounting brackets and light tunnels ofthe embodiment of FIG. 1;

FIG. 5a is a flowchart of a method for controlling particle velocity inaccordance with the present invention;

FIG. 5b is a flowchart of a method of calculating particle velocity inaccordance with the present invention;

FIG. 5c is a block diagram of a controller of the present invention;

FIG. 5d is a block diagram outlining particle velocity sensor softwareembodying the present invention;

FIGS. 6a to 6 d illustrate alternative air seeder configurations;

FIG. 7 is a block diagram of a controller in accordance with the presentinvention;

FIG. 8 is an exploded view showing an electric actuator on a tank forcontrolling the air damper position in accordance with the presentinvention;

FIG. 9 is an exploded view showing a hydraulic flow control value andfan speed electric actuator on the air cart frame in accordance with thepresent invention;

FIG. 10 is a hydraulic schematic diagram of an embodiment of the airflow control on a variable drive air cart in accordance with the presentinvention;

FIG. 11 is a front elevation view of the console and display panel forthe air flow control system, the console and display panel being locatedat a remote location in the prime mover; and

FIG. 12 is a velocity chart plotting monitor velocity numbers against anequivalent rate in pounds per acre.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1 to 12 depict aspects of an agricultural air cart embodying thepresent invention. The air cart employs a method of determining particlevelocity and a method of utilizing the particle velocity toautomatically control the air flow within the air cart.

If a particle or a group of particles passing one point can beidentified when passing another point, a know distance downstream, thena time measurement of the two events permits calculation of the particlevelocity. The illustrated embodiment relies on beam interruption toproduce a pair of signals that can be cross correlated.

Beam interruption sensors are applied to measuring the average velocityacross a diameter of a pipe. The sensor is placed in the air deliverysystem of the agricultural implement. The sensor includes two pairs ofoptical transmitter-receiver sets located in the same horizontal plane,with one pair being located downstream of the other. In one embodiment,at least one sensor is placed in each run with the sensor mounted justdownstream of the intermediate manifold.

The sensor design centres on a processing unit which must measure theoptical beam signals and then perform a number of multiplication andaddition functions on the signal data in order to calculate a crosscorrelation function. The particle velocity is then determined from thepeak location of the cross correlation function, and this value is usedto control the air flow of the air cart and can be output to the user.

A method of detecting the velocity of product being carried by air flowis disclosed. The method involves 1) a first beam being interrupted by agroup of particles, 2) a second beam downstream the first beam beinginterrupted by the same particles at a time Δt, 3) conversion of thevoltage signals from the first and second beams to digital signals, 4)cross correlation to the two signals in order to determine the time lagΔt, 5) conversion of the time lag to a velocity.

When multiple runs are active in an air cart, one run invariably is moreresistant to flow and particles will drop out of the air flow in thatrun first as the air velocity is reduced. In one embodiment, the averageparticle velocity is determined in each active run of the air cart. Thisdata is then used by a control system to control the air cart or isdisplayed to the user on a console unit. The method involves 1)determining the average particle velocity in each run, 2) transmittingthis data to a controller, 3) normalizing the velocities, 4) monitoringthe system to ensure each bank of runs operate at a minimum particlevelocity near the sustainable normalized value.

In the illustrated embodiment, a typical air cart unit 10 consists of afirst storage tank 12, second storage tank 13 and a third storage tank14, metering mechanisms 22, 23, 24, a variable speed fan 2, air cartelectronic control (not shown), an air distribution system 3, andpossibly earth working tools 4. The air distribution system 3 includes aseries of air flow tubes (runs) arranged in respective banks extendingfrom each tank. The electronic control provides electrical control for ametering mechanism corresponding to each of the storage tanks: a firstmetering mechanism 22 for the first storage tank 12, a second meteringmechanism 23 for the second storage tank 13, and a third meteringmechanism 24 for the third storage tank 14.

Air flow generated by the fan 2 travels through the tubes, and seedand/or fertilizer is metered into the tubes through the meteringmechanisms. The tubes are associated with a plurality of planting toolsto deliver the metered material by the flow of air for distributing onthe ground. This could be either with use of earthworking tools or abroadcast method.

In one embodiment, each bank of runs is provided with an air damper 25,26, 27. Each air damper is operable to restrict the flow of air in eachbank of runs as function of the rotated position of the damper between afully open position (unrestricted) to a fully closed position(restricted).

Particle velocity sensors are ideally located in each run of the airdistribution system.

In the preferred embodiment, the particle velocity sensor 40 comprisestwo pairs of optical transmitter-receiver sets, processing unit andmicroprocessor. Ideally, the transmitters are light emitting diodes andthe receivers are near infrared phototransistors. The transmitters aredriven with a DC signal and the receiver signals are AC coupled andamplified. The amplification value is such that a reasonably largesignal is provided from the smallest particle expected to be used inthis system. The optical devices selected result in minimum beamseparation in order to obtain accurate particle velocity values.

Preferably, the sensors are placed such that a transmitter of one pairis located adjacent to a receiver of the other pair to minimize opticalsignal coupling between the transmitter of one pair and the receiver ofthe other pair.

Preferably, the sensors are located just above the centerline of the airflow tube cross section.

Preferably, light tunnels 41 are used to reduce the possibility of lightfrom one transmitter being reflected from a passing particle to theadjacent receiver. The light tunnels are preferably configured such thattheir depth is sufficient to prevent a receiver from detecting lightreflected from particles. The light tunnels are illustrated in FIG. 4.The bracket is mounted over an optically transparent section of tubethrough which the particles pass.

The sensors are held in position by mounting brackets 42. The mountingbracket 42 is ideally of an optically opaque PVC plastic for ease ofmachining. The brackets are illustrated in FIG. 4.

During operation, product is metered from the centralized hopper intothe air stream supplied by the fan. The product entrained air travelsthrough the air distribution system. In order to control the system, aparticle velocity sensor is mounted in each run of the air distributionsystem. Note that in an embodiment in which particle velocities aresimply displayed and there is no automatic control of air flow, it isnot necessary to have a sensor in each run. A transmitter-receiver pairis mounted in the exterior of the air distribution tube with a secondpair located a fixed distance L downstream. Both pairs are in the sameplane. The transmitters and receivers are mounted at the end of smallholes drilled in the mounting tubes to collimate the light. Productpassing in front of the first transmitter-receiver pair interrupts thefirst light beam, and a time Δt later interrupts the second light beam.The inputs arc monitored for transitions indicating beam breakage. Whenthis occurs, the input data is buffered and a cross correlation isperformed.

The cross correlation of the optical sensor signals is implemented usinga microprocessor. This system consists of a microprocessor board withassociated memory and two input (one for each phototransistor signal)and two output analog channels.

FIGS. 5a to 5 c illustrate the method employed by the illustratedembodiment to monitor and control the particle velocity. At Step 105,the method begins upon activation by the operator of the air cart. AtStep 105, the velocity of particles in the conduit is calculated. AtStep 115, it is determined whether the velocity is below a suitablelevel. If the velocity is above this level, the velocity of theparticles is adjusted at Step 120 by adjusting the air flow. The abovesteps are repeated so that the particle velocity is continuouslymonitored and adjusted, until the process is terminated by the operator(Steps 125, 130).

FIG. 5b illustrates the method employed in Step 110 of FIG. 5a tocalculate the particle velocity in the conduit. At Step 205, particlesare detected at an upstream location in the conduit. At Step 210, anupstream signal corresponding to locations of the particles in theconduit at the upstream location is generated. At Step 215, particlesare detected at a downstream location in the conduit, the downstreamlocation being a known distance from the upstream location. At Step 220,a downstream signal corresponding to locations of the particles in theconduit at the downstream location is generated. At Step 225, the timeinterval between detection of the particles at the upstream location anddetection of the particles at the downstream location is determined bycross correlating the upstream and downstream signals. At Step 230, theparticle velocity is calculated by dividing the known distance by thetime interval.

FIG. 5c is a block diagram of the controller 305, which includes: aprocessing unit 310 to receive signals from the upstream and downstreamsensors; a calculator 315 to calculate a velocity of particles in theconduit; and an adjuster 320 to adjust the flow of air in the conduit asa function of the velocity.

The method includes sampling of the two analog input channels, checkingif the sampled signals indicate that a particle (or particles) havepassed the sensor, performing a cross correlation on the signals andcalculating the lag time to the peak of the correlation signal. Thecross correlation function is found by calculating a series of vectordot products, each with a successively large lag time applied to thesignal from the second optical sensor. Effectively, the second signal isshifted in time until the two signals are aligned, at which point theproduct of the two signals is maximized. This time shift represents thetransit time of a particle from the first optical beam to the second.

When both optical signals contain points above a minimum thresholdvalue, the starting point of the first signal in the data buffer isdetermined. From this point, the cross correlation function isdetermined using a set correlation length. Once the correlation functionis calculated, the lag time of the peak value in the function isdetermined. This is done by finding the two points in the function curvewhere the value is 95% of the peak value, and then calculating themidpoint of these locations. The purpose of this calculation is toaccount for cases where the top of the correlation curve is very flat(clipped), which occurs when both optical signals are clipped but ofdifferent widths.

There is some variation in signal amplitudes and time delays due to therandomness of the particle motion. Therefore, a number of filteringmethods have been implemented in order to smooth the results. A simplelow pass filter is used to process the individual particle velocityreadings into a running average value. The selected algorithm has aminimum threshold value that a beam interruption signal must exceed inorder to be processed into a velocity calculation. As an alternative tousing thresholds on both signals, after the cross correlation has beenprocessed its peak value must exceed a minimum threshold in order forthe associated velocity to be used in the filter calculation. Inaddition, if an individual calculated particle velocity deviates fromthe current running average by more than a present threshold value, thatreading is also rejected.

With reference to FIG. 5d, the main loop of the algorithm used toperform the cross correlation reads the results of the AD conversionafter the beams are interrupted (step 605) and then checks a number ofstatus flags to determine the next action to be taken. The first flagchecked is trans_flag (step 610), a flag that indicates whether a signaltransition has already occurred on the first optical beam signal(channel 0). If a transition has not yet occurred, the signal level ischecked from the last channel 0 reading to see if the transitionthreshold has been exceeded (step 615). If it has, then another flag(first_sample_flag) is checked to make sure that this isn't the firstsample after a previous particle signal has been processed (step 620).The reason for this is to ensure that a complete particle passage ismeasured, as opposed to starting a measurement when a particle isalready in front of the first beam. If only a partial beam breakagesignal is recorded, the resulting correlation measurement can be poor.

If a transition has occurred and this was not the first sample afterprocessing a previous particle, the transition flag trans_flag is set sothat the data buffer will start to fill on subsequent AD conversions(step 625). In addition, a flag called no_particle_cycles is reset (step625). This flag is used to limit the time that the sensor output canremain at a constant value with no new particles being measured. EveryAD conversion that occurs when the channel 0 value is below thetransition threshold causes this counter to be incremented (step 635).If it reaches a large enough value (representing approximately 0.2sec.), then the sensor output is set to zero, which indicates that noparticles are being detected (step 640).

If the transition threshold has not been exceeded and trans_flag was notset, then the last channel 0 reading is copied into a circular buffer(step 630). This allows readings before the transition point to beincluded in the cross correlation calculation and prevents useful datafrom being truncated. The first_sample_flag is reset since the signallevel is below the transition threshold and the no_particle_cyclescounter is iterated since no particle has yet been detected. Ifapproximately 0.2 sec. has passed with no particles detected (i.e.:no_particle_cycles exceeds a preset threshold), then the sensor outputis set to zero (step 640). Also, AD channels 2 and 3 are read todetermine the DC level of the optical beam signal through the window inthe pipe wall. Higher values indicate greater window blockage, so thetwo readings are compared and the larger of the two is output to PWMchannel 1. Also in this code section between particle samples, thewatchdog timer is reset if the watchdog timer counter k has exceeded apreset threshold.

If the transition flag trans_flag had already been set in a previousiteration, then the AD data from both optical beam inputs is copied todata buffers and the data counter index (ind) is incremented (step 650).Also, the signal level from the second optical beam is checked to see ifa particle breaks only the first beam and not the second, then itssignal will not be processed. The amount of code in the loop that isexecuted after a transition has been detected is kept to a minimum inorder to maximize the effective sampling rate. The counter index ischecked to see if the required amount of data for cross correlationprocessing has been copied to the data buffers (step 660). If so, thench1_ok flag is checked to verify that the second optical beam was brokenin this data set and the buffer_ready flag is then set (Step 665). Anumber of other flags are reset for subsequent iterations and a counteris incremented to count particles for spacing out future updates of theoptical window blockage signal output (Step 670).

If the buffer_ready flag has been set (Step 675), then the correlationfor the last particle passage will be calculated (Step 680). The firststep in processing the cross correlation of the beam signals is tore-order the circular buffer data at the start of the data buffer forchannel 0 (Step 685). Then, instead of computing a complete crosscorrelation for the entire length of the data buffers, a peak searchingalgorithm is used in an effort to minimize the calculation time requiredto determine the peak cross correlation value. Using the peak lag valuefrom the previous iteration as a starting point (set as cur_laginitially), individual cross correlation calculations are carried out atthis lag and at this value minus one. This is done using a functioncalled get_results, which scales the calculation and generates a valuecalled result and a value called carry (effectively a scale factor).Another function called cmp_result performs a comparison of the twoindividual cross correlations. Depending on which of the two results islarger, the algorithm then sets the cur_lag value to go in the directionof increasing results. This process is repeated until the result stopsincreasing and a decrease is found. At this point, a local peak in thecross correlation function has been located. The search iterationcontinues only to preset maximum and minimum lag values if no peak isfound.

Once the peak lag value has been found, an interpolation is carried outin order to refine the measured value. This is done because the limitedsampling rate results in a somewhat coarse measurement resolution. Theinterpolation involves the use of the individual cross correlationvalues just above and below the peak result. The amplitude differencebetween the peak and left value is compared to the difference betweenthe peak and right values. If the right difference is smaller, then thetrue peak is to the right of the calculated value. An interpolated lagat the new peak value is found by shifting the peak based on themagnitude of the left and right differences. In the calculation, the lagvalue is also scaled up for improved resolution and calculationaccuracy.

The interpolated lag value is then processed in a filtering algorithm,which also rejects individual readings which deviate more than a presetamount from the most recent filtered result value (Step 690). Only amaximum of two consecutive values can be rejected before subsequentvalues will be incorporated into the filtering calculation. Thisprevents sudden large deviations in the actual particle velocity fromfreezing up the calculation of new values. The filtering algorithmitself is a simple weighted filter where the filter output value is aweighted average of the current interpolated measurement and theprevious filtered output values. By increasing the weighting on theprevious filtered output, the filtering effect is increased. Once thefiltered lag value has been determined, it is converted into a valuebetween 0 and 255 for output to PWM channel 0. This involves dividing ascaling factor by the lag value. This scaling factor effectivelyrepresents a spacing between the optical beams divided by the samplingrate, all scaled by a factor to convert the output to 2 volts=1000 fpm.Because each of the optical sensor mounts has a slightly differenteffective beam separation distance, there is a different scaling factorfor each one.

A concern with the use of an optical technique for particle velocitysensing is the potential for obstruction of the optical window throughthe pipe wall due to scratching or coating with dust or othercontaminants. In order to provide for monitoring of this problem, thesecond PWM analog output of the CPU board is used to output a signalproportional to the DC level of the received signal at thephototransistor. As the window blockage worsens, this signal increases.The sensor will continue to operate normally until the effects ofparticle blockage of a beam are too obscured by the window blockage. Thepoint at which this occurs would depend on the type of product beingmeasured.

Initially, the window blockage signals are read and compared with astored minimum value of each signal. If a new reading is less than thestored value, then it replaces the stored value. The purpose of this isto attempt to compensate for the fact that the DC reading of the opticalvalues are affected by the passage of particles. The readings are takenat random timings relative to the passage of particles, so the valuesread will be at or above the values expected with no particles passing.When particles are detected, the window blockage signal is updated onlyevery 200 particles and the blockage signals are read twice per particlepassage. Therefore, the minimum value out of the 400 readings is used torepresent the effective blockage signals. This will generally be asituation in which no particles are in either of the two optical beams.After the filtered lag value has been determined, another check of thewindow blockage signals is made, and then if 200 particles have pastsince the last update, another window blockage signal value iscalculated and output to PWM channel 1.

The particle velocity sensing apparatus is intended to operate over arange of product sizes from clay carrier with an average diameter ofless than one millimetre to peas with an average diameter of sevenmillimetres. To register an interruption, the particle diameter mustcover a significant area of the beam. The algorithm is programmed suchthat the set correlation length is sufficient to cover the entirepassage time for the largest product diameter at the lowest anticipatedparticle velocity.

Another limitation factored into the design is that the physical size ofthe transmitters and receivers will dictate the minimum separationbetween the pairs. In general, the closer the pairs the better theoptical correlation. However, the minimum separation is dependent onsensor diameter and/or the required mounting hardware. A largerseparation will provide poor correlation but will also result in reducedsampling rates for the same velocity measurement resolution andaccordingly will reduce the demands on the processing system. Theoptical devices used are selected partially on the basis of diameter inorder to obtain minimum beam separation.

At least one sensor is provided in each run of the delivery system. Thelocation downstream or upstream in the run has little impact on systemperformance. Ideally, the sensor is mounted just downstream of theintermediate manifold.

The particle velocity sensors can be utilized in a number ofapplications. The data provided by the sensors is useful for informingthe user of the average particle velocity, in automatic control of theair system and in allowing automatic monitoring and control of optimumflow when more than one product is applied. Another feature of thedescribed invention is detection of blockage or partial blockage of seeddelivery lines by monitoring the optical signal levels electronically.Blockage of the delivery lines will result in a decrease in particlevelocity. The control system operating on the basis of the opticalsensor's particle velocity readings will detect this decrease andcounter it with an increase in air velocity and/or inform the users.

In order to utilize the particle velocity sensor for automatic controlapplications, a method of utilizing the velocity information for controlpurposes is necessary.

Data from the particle velocity sensors is communicated to an automaticair flow control system. The data is used to determine the lowestallowable fan speed, adjust the air damper, monitor and automaticallycontrol the air flow. This eliminates the need for user intervention inadjustments of the air flow system.

Different sizes of product flow at different minimum carry velocities,and also different mass flow rates of the same product flow at differentminimum carry velocities. Given the large range of product types andflow rates which must be handled by an air seeder, it is impractical tohave tables of all possible combinations of these to use as a basis forcontrol. In addition, when product is introduced into an air stream, itwill accelerate for a considerable distance. Given that, for each tankon an air seeder, the sensor mounting locations are at differentdistances downstream from the metering locations, it follows that theparticle velocities at the sensor locations will also be different.

The method used to overcome these problems is to use averaging andnormalization of particle velocities. What this means is that for onebank of delivery lines the average of all of the particle velocities forthose runs is calculated. Then for each run, its individual velocity isdivided by the average. So if a run is close to the average velocity, anumber close to one is found. If it is faster than average, the numberwill be higher than one and vice versa. With a high enough air velocity(ensuring that all product is flowing without blockage or impendingblockage), it was found that this ratio is consistent for everyindividual run. As the air velocity is increased or decreased over rangeof value, this ratio remains fairly constant. The nominal value of thisratio for each run is found, and dividing the measured ratio by thisnominal value results in a normalized relative velocity number.

This normalized relative velocity number is significant because itprovides an indication of the relative particle velocity in a run. Asthe air velocity is reduced, the normalized relative velocity numberstays constant (near 1) for all runs until slugging or impendingblockage begins. When slugging occurs, it typically starts in one or tworuns which have the highest resistance to flow. The relative velocitynumber in these runs then begins to decrease, giving an indication thatblockage is impending and that the air velocity should be increased toprevent blockage.

It is irrelevant that the absolute velocity in one set of runs may bedifferent than the absolute velocity in another bank of runs when thenormalization process is used. Control is based on the variationsbetween runs in the same bank from the same meter. A relative dropdetected in one or two runs is indicative of slugging, which can then becorrected by increasing the air flow in that particular bank of runs toprevent blockage.

For example, consider four tubes delivering product from a meter on anair cart. With the air flow initially set high enough to ensure that theproduct is flowing smoothly, the particle velocities measured could be:

V1=1200 fpm (particle velocity in tube 1, 1200 feet per minute)

V2=1150 fpm

V3=1050 fpm

V4=1000 fpm

The average particle velocity can then be found:

Va=1100 fpm

The velocity ratio in each run relative to the average value can then becalculated:

V1/Va=1.091

V2/Va=1.045

V3/Va=0.955

V4/Va=0.909

These relative values are then used to normalize any subsequentlymeasured velocity ratios in order to arrive at the relative velocitynumber (RVN):

RVN1=(V1/Va)/1.091

RVN2=(V2/Va)/1.045

RVN3=(V3/Va)/0.955

RVN4=(V4/Va)/0.909

Say the air velocity is now decreased slightly and the particlevelocities all decrease in equal proportion:

V1=1080 fpm

V2=1035 fpm

V3=945 fpm

V4=900 fpm

Now, Va=990 fpm and:

RVN1=(1080/990)/1.091=1.0

RVN2=(1035/990)/1.045=1.0

RVN3=(945/990)/0.955=1.0

RVN4=(900/990)/0.909=1.0

The RVNs are 1.0 and remain near this value as long as the particlevelocities all decrease in equal proportion. If the air velocity isreduced enough, typically one or two runs will begin to havedisproportionately large particle velocity decreases. This indicates theonset of slugging in the flow. This might appear as:

V1=972 fpm

V2=932 fpm

V3=800 fpm

V4=810 fpm

Now, Va=878.5 fpm and:

RVN1=(972/878.5)/1.091=1.01

RVN2=(932/878.5)/1.045=1.02

RVN3=(800/878.5)/0.955=0.95

RVN4=(810/878.5)/0.909=1.01

From this data, it is apparent that run 3 has had a much higher thanaverage decrease in its particle velocity and that the flow may be (ormay soon be) slugging. The control algorithm would detect that run 3requires a boost and increase the air flow in all banks of runs for thismeter until the RVNs stabilize near 1 again. The controller will checkthe average particle velocity when that critical point is reached andone RVN starts to drop and it would maintain the flow around thatvelocity or a bit higher. The limiting RVN would be checkedperiodically. If the product flow rate is changed by the user, then thesystem will again check the limiting RVN also.

In order for the adjuster (Step 120 in FIG. 5a) to automatically controlair velocity, an air cart preferably has controls on the fan speed and amethod of proportioning the overall air flow across the individualmeters. Preferably, the air cart unit is outfitted with electricactuators to control fan speed as well as dampers in each of the airdelivery runs. These actuators are electronically controlled with aremote unit which communicates via the communications bus. Commands tochange the fan speed or individual damper positions as well as data oncurrent fan speed are communicated on the communications bus.

The goal in setting the fan speed and damper positions is to attain theminimum product carry velocity in the flow from each of the meters. Oneskilled in the art will realize that the air flow speed throughout theair distribution system is important in maintaining good farmingpractices. Planting without adequate amounts of air in each set of runswill result in blockage of seed and debris. An excess of air flow in theruns will result in scattered patterns of seed placement in furrow dueto seed bounce. Excess air flow velocity can also impact and damageseed. The control system of the present invention uses particle velocitymeasurements in each run to determine the minimum carry velocity foreach set of runs, and will send out commands on the go to adjust fanspeed and damper positions in order to maintain this minimum carryvelocity. The control system includes its own electronic control unitwhich will be connected to the particle velocity sensors. This unit willprocess the velocity signals, perform the calculations for determiningthe minimum carry velocities, and send out commands for adjustments tofan speed and damper positions and display the results continuously tothe user.

Air seeders can commonly include up to three or more tanks forapplication of up to three or more different products and three or moregroups of delivery lines in the air distribution system. Each bank ofruns will have a different optimum air flow based on the particularproduct, assuming multiple products are not blended into a single run.The goal is to obtain optimum particle velocity in each of three banks.Complications lie in the fact that there is only one air source for allthree runs. Therefore, adjustments of fan speed or of the air damper inone run affect the air flow in all runs. Again with the use of particlevelocity sensors, the goal is to automatically monitor and control fanspeed and damper positions to maintain optimum particle velocity in eachrun even as particle flow rates change. Alternatively, particle velocityis continuously monitored to maintain optimum particle velocity in eachrun even as particle flow rates change.

However, air carts common in the industry allow for single shoot (FIG.6a), double shoot (FIG. 6b and 6 c) and triple shoot (FIG. 6d)applications as well. In single tank, single shoot applications, productis being delivered from only one tank. Only the fan speed would need tobe controlled in order to adjust particle velocity. The damper for theactive meter would be fully open and the other dampers would be closed.Multi-tank, single shoot applications involve blending products from twoor more tanks into one run. The particle velocity sensors would belocated after the point where the products are blended, so they will bemeasuring the velocity of the combined flow. The damper position of themeter with the higher product mass flow rate would be fully open, andthe fan speed and the other damper position would be controlled. Doubleand triple shoot applications consist of separate product flows from twoor more tanks right to the opener. In this situation, the damper fromthe high mass flow rate product would be fully open, and the fan speedand the other damper position (or positions in the case of tripleshooting) would be controlled to balance the air flow amongst the runs.

A combination where two tanks are blended into one set of runs andproduct from a third tank travels in its own set of runs is also acommon configuration used in the industry.

In multi-tank single shoot applications, products of different carryvelocities travel in the same run. The larger product may drop out ofthe flow (below the sensor's field of view) and begin slugging while thelighter product could continue to flow over the top and thus mask theslugging going on. Two methods of countering this problem areconsidered. In one implementation, the correlation algorithm detects andprocesses the velocities of different sized particles separately.

By tracking the velocity of the heavier particle flow, it can also becontrolled and thus prevented from dropping below the field of view ofthe sensor. Also, if its absence was detected by the sensor, a warningcould be transmitted over the communications bus indicating a lack ofproduct flow. A second implementation is the use of a “ramp”. The rampis placed immediately before the sensor at the bottom of the tube. Itsfunction is to force particles moving along the bottom of the tube upinto the sensor's field of view allowing their velocity to be measured.

In operation, metering of product would start at a high air rate inorder to determine initial average velocities and relative velocitynumbers (RVNs). The initial air flow rate would be based on product typeand mass flow rate. Individual velocity readings would be filtered witha low pass filter in order to minimize the effects of the distributionof particle velocities for a given average velocity condition. Somefiltering is carried out in the sensor microprocessor in order to attaina reading which can be tracked for sequential calculations of particlevelocity. Further filtering and then the averaging and ratiocalculations of individual velocities are carried out in the remotecontrol electronic unit.

In the preferred embodiment of the control algorithm, RVNs are useddirectly, the system continuously monitors all runs and increments ordecrements fan speed and damper settings to try to maintain RVNs justabove a threshold value. At a given product flow rate, the criticalaverage particle velocity is found to just maintain the minimum RVNabove a threshold value. Average particle velocity means the averagevalue across all runs in a bank for a particular meter. This criticalvelocity is used as a short-term set point for the control system. Useof a set point slightly above this value allows for delays in thecontrol action and provide more margin of safety against plugging. TheRVNs would also periodically be monitored in case adjustments in theparticle velocity set point become necessary. When product flow rateincreases, this will show up as a velocity decrease and the controlsystem will counter this with an increase in air flow. This will alsoaffect RVNs and the controller will continue to increase air flow as itadjusts for a new set point.

In the single tank, single shoot configuration, the active tank's damperis in the fully open position and the other dampers are closed. Once theinitial air and product flow rates are set and the RVNs have beenestablished, fan speed is decremented and conditions are allowed tostabilize for a fixed time period. The RVNs are then checked for anyvalues which have decreased to below a preset threshold value. Based onthe filter characteristics, a minimum number of consecutive readingsbelow the threshold are required in order for the system to consider thethreshold broken. If there are any tubes with a RVN below the thresholdvalue, the last fan speed decrement is reversed and the previous settingof the fan speed is selected. This condition is monitored for a fixedtime to ensure stability. If the lowest RVN or RVNs are still below thethreshold, the fan speed is incremented again and the steady statecondition is again checked. Once a stable operating condition isattained, the average particle velocity and the product flow ratesetting are noted. As long as the product flow rate setting remainsconstant, the average velocity value measured acts as a short term setpoint for controlling the air flow. The fan RPM is increased ordecreased based on short-term variations in the average particlevelocity value. The RVNs are continuously monitored to ensure that nonedrop below the threshold. Periodically, the set point is retested bydecreasing the fan speed and checking the RVNs for any significantdecreases. If a fan speed adjustment is required based on a change inthe RVNs, a new average particle velocity set point is found once astable operating condition is again reached.

For the case of single shooting product from one tank, only the fanspeed control described above would be required. For single shootingfrom two tanks, the damper from the higher product flow rate meter wouldbe wide open, and the other damper would be set for an air flow inproportion to the relative mass flow rate of product from its meter.

In double or triple shooting, when one meter gets an increase in airflow with an increment in damper position, the air flow to the othermeters will decrease initially. The control system will detect this asthe average particle velocities begin to decrease in the runs with thedecreasing air flow. This would be compensated for by incrementalincreases in damper position and fan speed. For the meter with thehighest product flow rate, the damper would be fully open and controladjustments are applied directly to fan speed. So when there is adifference between the set point value of the average particle velocityand the current measured value (i.e. an error in the velocity), a changein fan RPM is commanded by the control system. Typically the magnitudeof this change would be proportional to the error. Depending on thecharacteristics of the system, it may be determined that the change infan speed should be related to the magnitude of the error, its rate ofchange, and/or its accumulated magnitude over time(proportional/derivative/integral control). The parameters used for thecontroller would be tuned to suit the characteristics of the system.

The above description also applies to the case of adjusting an airdamper position to change the air flow rate (and thus particle velocity)in the runs from a given meter. The amount the damper is open or closedwould be proportional to the error in the particle velocity (or possiblyalso related to the rate of change of the error or its accumulatedmagnitude over time). The position of the linear actuator operating thedamper would be converted to a function which provides a reasonablylinear change in air flow output for a given input. In other words, thecommands to the damper actuators would be for air flow changes ratherthan simply actuator position changes.

The input commands from the above mentioned algorithms are thentransmitted from the particle velocity remote unit to the air cartremote by a conventional communications bus as seen in FIG. 7. Thecircuitry consists of a microprocessor, memory unit and driverelectronics. One driver electronics circuit is required for eachelectric actuator. The preferred embodiment for the control system isfor use on air carts with three air dampers and one fan, thus requiringfour electric actuators, however, an alternative embodiment could usemore or fewer actuators within the scope of the invention.

The microprocessor is configured to receive data inputted from thecommunications box and use this data to control the driver electronics.The driver electronics used in the air control system are such that theyexperience low power loss. The memory serves as a means of programstorage and data storage. The microprocessor, memory unit and driverelectronics are all standard components.

Each tank on the air cart unit is outfitted with one air damper electricactuator used to position the air damper of that particular tank. Theair damper electric actuator 32 is bolted to the storage tank 12. Therod end of the linear actuator is bolted to an electric actuator controlarm 39. The control arm is then pivotally mounted at a connection jointon the air cart frame 11 and is attached to the air damper 25. The otherair damper electric actuators are mounted in a similar manner. Each ofthese air damper linear actuators is also connected to the air cartelectric harness by an independent extension harness (not shown). Theactuator control arm is bolted to the electric actuator and is pivotallymounted to the air cart frame. It is also attached to the air damper.Movement of the electric actuator and control arm results inproportional movement of the air damper, thus positioning it to achievethe desired air flow rate.

The fan speed linear actuator 30 is mounted to the hydraulic flowcontrol valve 35, while the flow control valve is mounted to the aircart frame 11 with a mounted plate 31. The fan speed electric actuatoris also connected to a flow control actuator arm 33. An extensionharness is used to connect the fan speed electric actuator to the aircart electric harness (not shown).

Alternatively, fan speed may be controlled by controlling the speed ofan engine driving the fan, or controlling the displacement of a variabledisplacement pump supplying hydraulic flow to the fan.

The hydraulic circuit is the preferred embodiment for a variable drivecart. The hydraulic flow control valve 35 is shown to have an “IN” port,a “CF” port and a “EX” port. The hydraulic fluid enters the hydraulicflow control valve via the pressure manifold 52 through the “IN” port.The control flow exits via the return manifold 53 through the “CF” portand the “EX” or excess flow port is fitted with a plug. The fan speed isadjusted by positioning the fan speed electric actuator which sets thehydraulic flow control valve and thereby sets the fan speed.

The entire system is operated by the user via a console unit located inthe tractor cab. The console serves as the command means for the user(user inputs product flow rates, etc.) and also serves as the outputmeans for the system (system outputs RVNs, system errors, etc.). Withthe RVNs being displayed on the console, the user is informed which runor runs are most resistant to flow and the flexible plastic hoses of theair distribution system may be configured to minimize that resistanceand make the cart more efficient.

In variable rate application, the data obtained from particle velocitysensors can be used in conjunction with the automatic control system.Variable rate application is important because each area of the land isof a different composition, thus requiring different rates of productapplication. With current manual air flow control, it is impossible tocontinually adjust the air flow to optimum values as the product flowchanges. However, with particle velocity sensors and automatic controlin accordance with the present invention, the system could continuallybe kept near optimum values.

There are also a number of different particle velocity sensorembodiments which would improve optical signal strength throughpartially obscured windows. One option is to use pulsed instead ofcontinuous optical transmission signals. This allows for much higherintensity drive signals to the transmitting LEDs because the shorterbursts of drive current can be of much higher amplitude, although theaverage power dissipated by the device remains the same or lower thanwith continuous operation. This also allows for synchronization so thatthe two transmitting LEDs are never on at the same time. In this way,the transmitters may be located adjacent to each other, simplifyingcircuit design and reducing the need for tunnels for the optic devices.

Another embodiment of the particle velocity sensor would utilize drivesignal amplitude control, used to maintain the minimum amount ofintensity required for the receiver to “see” the light transmittedthrough the windows. When windows are clear, the minimum amplitude ofLED transmitter drive current pulse is used which prevents the receiverfrom being saturated with light when small particles may be in the beamand should be detected. As the sensor window becomes more obscured,higher LED current pulse amplitudes are used to ensure that the opticalreceiver sees the transmitted signal.

A third embodiment uses receiver gain control, used also as describedabove to prevent saturation and to allow for more sensitivity as thewindow becomes more obscured.

There are also other algorithms for the automatic air flow control. Asecond method involves the basic function of determining the minimumparticle velocities to maintain the RVNs above a threshold value similarto that of the first method. The difference involves the method ofresponding to changes in product flow rates. Instead of responding toerrors in particle velocity as a product flow rate change takes place,commands for product flow rate changes would be monitored by the controlsystem.

When a change in product flow rate is sent out on the CAN bus, the airvelocity control system would begin to respond to it at the same time asthe product metering system. Using a simple model of the air system on acart, an initial adjustment in the air flow rates is made for all activeruns. This adjustment is proportional to the change in mass flow rate ofproduct in each run. After this initial adjustment, the control actionas described in Method 1 would be used to seek out the new averageparticle velocity in each set of runs which resulted in operation justabove the RVN threshold.

A third method is similar to the second method, but instead of using amodel of the air system to arrive at approximate air flow adjustments tocarry certain product mass flow rates, the model is based on averageparticle velocities required to carry certain mass flow rates. In thiscase, when a command for a change in product flow rate is detected bythe control system, it immediately starts adjusting fan speed and damperpositions in order to attain a change in average particle velocity foreach meter proportional to the change in product mass flow rate for thatmeter. Once the fan speed and damper position adjustments stabilize, thesame control action as described in the first method would be used tooptimize the air velocities.

Among the advantages achieved are the following. Means are provided forautomatically monitoring and controlling fan speed and dampingmechanisms to maintain the system at optimum air flow settings on thego. The method of controlling an air cart does not require userintervention after initial settings are made. The system can sensevarious products and a wide range of velocities and control air flowregardless of products and configuration used. The means of maintainingcorrect air velocity levels in an air cart is simplified. Embodiments ofthe invention can automatically maintain optimum air flow settings tocarry products without blockage while lowering power requirements. Thereis no need for a separate system to monitor blockage in the banks ofruns, empty tanks or product bridging in a tank. Embodiments of theinvention can continuously monitor air flow settings to ensure productsare being carried without blockage while lowering power requirements.

In one embodiment, wherein there is no automatic control of the airflow, the apparatus for controlling the flow of air in the air cart unit10 includes a console 40, typically placed in the cab of the primemover, such as a tractor (not shown), to which the air cart unit 10 isconventionally coupled to provide operative and motive power to the aircart unit 10. The console 40 and display panel 42, from where fan speedand damper settings are controlled, is depicted in FIG. 11. The console40 has a keypad 45 for data entry and scrolling. Included on the keypadare the on/off key 46, the alarm key 47, and various selection keys 48.The increase 49 and decrease 49 a keys are used in setting the valuesfor fan speed and air flow speed. A display screen 43 is also on theconsole 40 to provide a means of output data to the operator.

The control commands are entered via the keypad 45 by the operator. Theoperator sets the desired fan speed using the increase an decrease keyson the console and display panel. The air dampers in each air flow tube21 are also set by using the increase and decrease keys on the controland display panel. A velocity number is determined by a method outlinedbelow with respect to setting the fan speed and is dependent upon thetypes of seed being planted, and whether a single, double or tripleshoot configuration is being used. The velocity is adjusted using theincrease and decrease keys on the control and display panel. Themanipulations of the increase or decrease keys effects adjustment of theposition of the linear actuator, which in turn will adjust the airdamper accordingly and demonstrate the resultant velocity change on thedisplay panel.

The input commands are then transmitted from the console unit 40 to theremote electronic circuitry 44, schematically represented by the airflow control remote block diagram of FIG. 7, by a conventionalcommunications bus. The electronic circuitry 44 is located within theremote unit. The circuitry 44 consists of a microprocessor, memory unitand driver electronics.

Once the system has positioned the air dampers 25, particle velocitysensors read the air flow rate and output the data on the display screen43. The particle velocity sensors are locate din the meter ports 28 oneach bank of tubes on the air seeder 10.

If the output data is not the desired value, then the operator simplyadjusts the value (either the velocity number or the fan speed) usingthe increase and decrease keys on the console unit.

The air control system is also equipped with an error/alarm signal. Thealarm key 47 on the console and display panel 42 is used to acknowledgeand recall alarms from the air cart monitoring electronics including airflow. A variety of error messages that appear on the display screen 43are available to inform the operator of any malfunctions.

To set fan speed, the operator would go through the following steps: setthe console so that it displays the fan speed screen; select the to prowof the display using the top select key; and set the desired RPM asdetermined by velocity charts common in the industry using the increase(+) or decrease (−) keys on the console unit.

Then to set air flow at each metering roll, the operator will go throughthe following steps: set the console so that it displays the air flowscreen; select the tank for which the air flow must be set; set thedesired air flow value by using the increase (+) or decrease (−) keys onthe console unit to set the velocity number as determined by the methodalready known in the art, which is to determine the “total rate”; thencalculate the equivalent rate, and then approximate the velocity numberfrom a known velocity chart, such as shown in FIG. 12; wait five tothirty seconds to ensure that accurate readings are made; and if the airflow reading is not the desired air flow value, the operator must repeatthe steps above until the displayed value is the desired air flow value.

The above procedure must be repeated for each tank for which the airflow must be set.

One skilled in the art will realize that the air flow speed throughoutthe air distribution system is important in maintaining good farmingpractices. Planting without adequate amounts of air in each set of runswill result in scattered patterns of seed placement in furrows due toseed bounce. Excess air flow velocity can also impact and damage seed.Therefore, prior art utilizes air dampers to control the air flowsettings throughout the air distribution system. These dampers arecommonly set by adjustment levers for each bank of tubes. Rotating thehandle either closes or opens the air dampers. With the presentinvention, the use of the adjustment lever is eliminated.

The operator first sets the desired value for fan speed (rpm). Aspecific fan speed value is recommended depending upon the number ofruns used and the type of product being distributed.

The increase or decrease command is, then, transmitted from the consoleunit to the remote electronic circuitry by the communications bus. Theremote unit receives the command and transfers the data to the driverelectronics which controls the hydraulics circuit. The hydraulic circuitincludes a hydraulic flow control valve and an electric linear actuatorused in controlling the fan speed. The position of the fan speed linearactuator positions the hydraulic flow control valve and sets the fanspeed rate.

The operator, then, sets a velocity number as determined by a simplecalculation defined above by using the increase and decrease keys on theconsole unit. A separate velocity number is entered for each set ofruns. Again, the increase or decrease command is transmitted from theconsole to the electronic circuitry by a communications bus. The driverelectronics receive the command and transfer the data to the driverelectronics which control the hydraulics circuit.

When an increase or decrease command for a particular air camper isentered by the operator in the console unit, each corresponding linearactuator is positioned. The actuator control arm is bolted to the linearactuator and is pivotally mounted to the air cart frame. It is alsoattached to the air damper. Movement of the linear actuator and controlarm results in a proportional movement of the air damper, thuspositioning it to achieve the desired air flow rate.

Once the linear actuators have set the fan speed and positioned the airdampers, velocity sensors read the air flow rate, which are transmittedto the console on the communications bus and then the data is output onthe display screen. This allows the operator to make adjustments to thefan and damper settings if the actual flow rate is not the desired rate.This process is repeated until the desired setting is reached for eachtank.

The output data from the velocity sensors also informs the operator ofany changes or fluctuations in the air speed during operation. In thisway, the operator is always informed of the present air flow rates andcan make any necessary changes on the go in order to keep the desiredfan speed and air flow rates during operation.

The air control system is also designed to send error/alarm signals tothe console unit. A variety of error messages are available to informthe operator of any malfunctions during operation.

While the present invention has been described in connection with whatis presently considered to be the most practical and preferredembodiments, it is to be understood that the invention is not limited tothe disclosed embodiments. To the contrary, the present invention isintended to cover various modifications, variations, adaptations andequivalent arrangements included within the spirit and the scope of theappended claims. The scope of the claims is to be accorded the broadestinterpretation so as to encompass all such modifications and equivalentstructures and functions.

Having thus described the invention, what is claimed is:
 1. In anapparatus for dispensing, the apparatus comprising: a. an airdistribution system comprising at least one tube for transporting airborne particles in a flow of air having a flow rate; b. an air flowsource for providing a flow of air to the air distribution system; c. acontroller operably connected to the air flow source for automaticallyeffecting an adjustment of the flow rate; d. a particle velocity sensorlocated at the air distribution system for providing a sensor outputsignal indicative of a velocity of air borne particles in the tube, theparticle velocity sensor including: i. an upstream light interruptionsensor, including an optical transmitter and an optical receiver,located at the tube to detect particles at a first location in the tube;and ii. a downstream light interruption sensor, including an opticaltransmitter and an optical receiver, located at the tube to detectparticles at a second location in the tube, the second location being ata known distance downstream of the first location; and e. wherein thecontroller is operably connected to the sensor and to the air flowsource, such that the controller automatically effects an adjustment ofthe flow rate by receiving a sensor output signal and automaticallyeffecting an adjustment of the air flow source as a function of thesensor output signal and a target particle velocity value.
 2. Theapparatus of claim 1 wherein: a. the air flow source comprises a fan anda fan drive mechanism operably connected to the fan to power therotation of the fan through a range of speeds; and b. the controller isoperably connected to the fan drive mechanism for automaticallyeffecting adjustments of the flow rate.
 3. The apparatus of claim 1wherein: a. the air distribution system further comprises a dampermechanism for selective control of the flow rate; and b. the controlleris operably connected to the damper mechanism for automaticallyeffecting adjustments of the flow rate.
 4. The apparatus of claim 3: a.further comprising a sensor located at the air distribution system forproviding a sensor output signal indicative of a value of a flowcharacteristic; and b. wherein the controller is operably connected tothe sensor such that the controller automatically effects an adjustmentof the flow rate by receiving a sensor output signal and automaticallyeffecting an adjustment of the damper mechanism as a function of thesensor output signal and a target value for the flow characteristic. 5.The apparatus of claim 4 wherein: a. the damper mechanism comprises: i.an air damper mounted within the tube for movement relative thereto, theair damper being operable to restrict the flow of air through the tubeas a function of the position of the damper relative to the tube betweenan open position wherein the flow of air is substantially unrestrictedand a closed position wherein the flow of air is substantiallyrestricted; and ii. a damper actuator operably connected to the airdamper to effect movement between the open and closed positions; and b.the controller is operably connected to the damper actuator forautomatically effecting adjustments of the flow rate.
 6. The apparatusof claim 1, further comprising: a. a storage tank for containingparticles to be dispensed; and b. a metering mechanism for transferringparticles from the storage tank to the air distribution system.
 7. Theapparatus of claim 1 wherein the transmitter is a light emitting diode,and the receiver is a near IR phototransistor.
 8. The apparatus of claim7 wherein the optical transmitter and optical receiver of the upstreamsensor are coplanar, and the optical transmitter and optical receiver ofthe downstream sensor are coplanar.
 9. The apparatus of claim 8 whereinthe optical transmitter of the upstream sensor is located adjacent theoptical receiver of the downstream sensor, and the optical receiver ofthe upstream sensor is located adjacent the optical transmitter of thedownstream sensor.
 10. The apparatus of claim 9 wherein the conduitcomprises light tunnels into which the transmitters and receivers of theupstream and downstream sensors are mounted.